I bought an MPC Supercharger kit from St Louis Music for my X340 around 1978 or so; it included new volume and tone control pots and some other electical stuff (resistors?); also included were Supercharger kit stickers for your control knobs; that is what is pictured - regular control knobs with the S/C kit stickers on them; personally I didn't like the way this looked so peeled them off and then stuck them on the back of the guitar on the black MPC compartment door, one on the left side, one on the right.

Note that the eBay article lists the guitar brand as Danelectro ..........